Vacuum assisted wound dressing

ABSTRACT

Apparatus for the application of topical negative pressure therapy to a wound site is described, the apparatus comprising: a wound contacting element for retaining wound exudate fluid therein; a wound covering element that provides a substantially airtight seal over the wound contacting element and wound site; a vacuum connection tube connecting a wound cavity to a vacuum source; and a vacuum source connected to a distal end of the vacuum connection tube.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 12/744,055, filed May 20, 2010, which is a U.S. National Phase ofthe PCT International Application No. PCT/GB2008/051088 filed on Nov.20, 2008, designating the United States and published on May 28, 2009 asWO 2009/066104, which claims priority to Great Britain PatentApplication No. 0722820.8, filed Nov. 21, 2007. The disclosure of theseprior applications is incorporated by reference in their entirety andshould be considered a part of this specification.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a topical negative pressure (TNP)assisted wound dressing particularly, though not exclusively, for thetreatment of wounds of relatively low area and/or relatively low volume.

The subject of this invention is an apparatus for the management ofsmall to medium sized wounds that utilises a vacuum source but manageswound exudate in a traditional manner by utilising an absorbentself-cohesive material in the wound cavity. No fluid is exported fromthe locality of the wound cavity other than by local evaporation. Inthis manner, an extremely portable system, of minimal inconvenience tothe wearer, can be generated.

Background to the Invention

TNP therapy has enjoyed relatively recent commercial success. WO9605873and its family members describes a portable TNP therapy apparatus. Theapparatus described mechanically supports tissue in the vicinity of thewound site and tissue mechanics and the rate of exudation from suchsites requires a system such as that described in WO9605873 or asdescribed in patents such as GB2378392 and WO2005/082435 which haveremote waste receptacles to which wound exudate fluid is aspirated inorder to cope with the volume of fluids generated in a relatively shorttime, less than that period in which a dressing would normally be leftin place. However, for wounds of surface area below approximately 200cm² or internal volumes below about 1000 cm³ these solutions may not bethe most appropriate since exudate volumes and exudate rates from thesewounds may be managed by more traditional wound dressings, requiringdressing change every 3-7 days. The relatively small dimensions of suchwounds do not make them attractive for the traditional TNP therapiesdisclosed in the prior art; these devices typically including a remotevacuum source, fluid transfer lumen and remote fluid collectionreceptacle, control and power source and are of dimensions and weightexceeding those convenient or discrete for the patient to carry.

The general principles of TNP apparatus described in the prior artcomprise a fluid-permeable wound cavity filling element, a dressing forcovering the wound and providing a reasonably air-tight seal around thewound, a drainage tube connecting the wound site and cavity fillingelement to the vacuum source via a fluid collection canister. Theprecise nature of the wound filling element has been the subject of muchinvention in this field. The mode of action of the apparatus is theapplication of negative pressure to the wound cavity, causingcompression of the wound cavity filler and expansion of the surroundingtissue into the wound cavity. Wound exudate is drawn from thesurrounding tissue, through the still porous cavity filler, along thedrainage tube and into the remote collection receptacle. An importantfeature of the prior art is the ability of the wound cavity filler toremain sufficiently porous, when compressed under negative pressure, toallow fluid transport from the tissue to the drainage or aspirant tube.Porosity can be facilitated at the molecular level, for example in ahydrogel, or at the microscopic level, for example in a hydrocellularfoam. To facilitate fluid flow, a hydrophobic filling has been deemedparticularly desirable by workers in the field and absorbent fillers asbeing particularly undesirable due to their hindering fluid transport.

In contrast to the principles of TNP therapy, the general principle oftraditional wound dressings is the localisation of wound exudate at thelocality of the wound, either within the wound cavity or in closeproximity to the surface. For this purpose, extremely absorbentmaterials are desirable that retard the free flow of fluid, preferablyabsorbing the fluid and localising it. Aquacel (trade mark) made byConvaTec Ltd is an example of a non-woven dressing that absorbssubstantial quantities of fluid and effectively locks it in thedressing. Allevyn (trade mark) made by Smith & Nephew Ltd is an exampleof a foam dressing that absorbs substantial quantities of fluid whileallowing rapid transpiration through a high moisture vapour permeabletop-film.

SUMMARY OF SOME EXEMPLIFYING EMBODIMENTS

In summary, the prior art deals exclusively with vacuum assisted fluidtransport away from the site of the wound. A very broad range of woundcavity filling and contacting elements have been described andexemplified in the prior art, including materials commonly used intraditional wound care dressings. Without exception in these cases, thecavity filling and wound contacting elements act as a conduit for thetransport of fluid from the wound per se to a remote collection canistervia an aspirant tube connected to a vacuum source.

An object of the present invention is to overcome or reduce thelimitations of the prior art for the management of wounds of low surfacearea, particularly those below approximately 200 cm² or internal volumesbelow 1000 cm³, while not resorting to the exclusive use of traditionalabsorbent dressings. A further object of the present invention is toovercome or minimise the problem of vacuum device portability.

According to a first aspect of the present invention there is providedapparatus for the application of topical negative pressure therapy to awound site, the apparatus comprising: a wound contacting element forretaining wound exudate fluid therein; a wound covering element thatprovides a substantially airtight seal over the wound contacting elementand wound site; a vacuum connection tube connecting the wound contactingelement to a vacuum source; and a vacuum source connected to a distalend of the vacuum connection tube.

The wound contacting element essentially blocks liquid transport beyonditself under pressures between atmospheric pressure and 200 mmHg belowatmospheric pressure. Preferably, the wound contacting element materialblocks liquid transport beyond itself at pressures of up to 250 mmHgbelow atmospheric pressure.

This invention concerns apparatus including a dressing for themanagement of wounds. In contrast to current therapies and prior art inthe field of TNP therapy, the invention provides a system that, whileexposing the wound to the many benefits of TNP, does not allow theexport of wound fluid from the confines of the wound cavity. Theapparatus relies upon a wound contacting element that does not allow thetransport of fluid beyond itself under the range of negative pressuresbeing applied to the wound.

A particular advantage of the apparatus according to the presentinvention is that confinement of wound fluid to the immediate vicinityof the wound enables the provision of an extremely small and light andconsequently highly portable vacuum source and a convenient simplecoupling and decoupling means of the vacuum source to the wound sitedressing and which overcome significant limitations in the usability andportability of prior art apparatus.

In the present specification the term ‘wound contacting element’ meansthe portion of the apparatus/dressing filling the wound cavity orcovering the wound. The nature of the wound contacting element is notrestricted, provided that its composition or structure is able toessentially block the flow of exudate away from itself under thepressure range specified above.

For example, the wound contacting element may include a liquid-triggeredvalve that closes when it becomes in contact with a liquid. Such a valvemay be situated proximate to the vacuum tube connection point at thewound covering element. Thus, when contacted by liquid the valve closesand no liquid is transported by the vacuum tube. The valve may be anelectromechanical valve or a smart valve comprised of a water-absorbentmaterial that expands to close within a constriction upon contact withliquid. The water-absorbent material may be placed within a restrictedaperture within the valve. The wound contacting element positionedbetween such a valve and the wound may be absorbent or non-absorbent butmay preferably be absorbent. The filling, in this embodiment, may be anymedically-suitable composition such as, for example, a gauze, a foam, awoven, a non-woven, knit or moulded material.

Alternatively, the wound contacting element may comprise entirely, or inpart, a suitable material structured such that it can absorb woundexudate but does not allow transmission of this fluid to the vacuumtube. This configuration is defined by two parameters: the aperture ofthe exit(s) from the material constituting the wound contacting elementand the mechanical integrity of the material when wet. For example, anabsorbent material that becomes laden with exudate must not itself bedisplaced along the vacuum connection tube. This is a particular problemwith particulate or fibrous so-called superabsorbent materials, for theycan pass, even when fully saturated with fluid, through very smallapertures of a size below which vacuum levels cannot be efficientlymaintained. In effect this means that the narrower the tube transmittingthe vacuum, the greater the loss in vacuum pressure with distance fromthe vacuum source, which loss in negative pressure is negligible formacroscopic bores but can be significant as bores reduce below 1 mmdiameter. To overcome these problems, an absorbent material withsubstantially enhanced self-cohesive properties compared to thosecurrently available is described in our co-pending patent applicationGB0719065.5 of common ownership herewith.

When an absorbent material is included in the wound contacting element,the absorbent material not necessarily acting as a fluid transportblocking element, the material may preferably be capable of absorbingmore than 5-times its own weight in fluid, more preferably more than10-times its own weight in fluid and more preferably still more that15-times its own weight in fluid. Such high w/w fluid absorbency may bedesirable so that the wound can be initially dressed with a low weightmaterial thereby reducing stress on the wound and the patient.

One group of materials particularly suited for this purpose areso-called superabsorbent materials, for example, those based onpolycationic or polyanionic polymers. Superabsorbent polyanionicpolymers include polyacrylic acid salts and polyacid derivatives ofpolysaccharides, such as carboxyalkylcellulose, or structuralderivatives. Preferably, when the material is polyanionic, it may be apolyacrylic acid salt or derivative or carboxymethylcellulose orderivative. Preferably, when the material is polycationic, it may bechitosan-based, more preferably a carboxyalkylchitosan or derivative,even more preferably carboxymethylchitosan.

One particularly preferred material is a superabsorbent material capableof self-coalescence upon fluid absorption (see our GB0719065.7 thecontent of which is included herein by reference). These materials areable to effectively block the transport of liquid beyond theirboundaries and also do not themselves flow or disaggregate under theinfluence of negative pressure or at the levels of externally appliedphysical stresses resulting from the negative pressure within a woundcavity.

A preferred material attribute may be the ability to achieve rapidhaemostasis in the event of bleeding in the wound site.

A further preferred material attribute may be the ability to killpathogens, such as bacteria or fungi, which come into contact with it.Preferably the material is inherently antimicrobial.

Carboxyalkylchitosan-based materials are suitably both haemostatic andantimicrobial.

The wound contacting element material can be provided in any formsuitable to enable fluid ingress and absorption but to block the flow offluid away from the wound contacting element. Suitable designs includedispersions of superabsorbent particles within a network of wickingfibres (as utilised in diapers, for example) or reticulated ordiscontinuous material comprising the superabsorbent material, such asopen celled foams, knits, laminates, woven or non-woven materials.Preferably, the material may be in the form of a non-woven sheet forapplication to largely two-dimensional wounds or non-woven balls forapplication to largely three-dimensional wound cavities.

The wound covering element may be any material substantially impermeableto the flow of liquid, but may or may not be substantially permeable tothe transmission of water vapour. The wound covering element ispreferably a highly conformable transparent material which mayoptionally be coated with, or be manufactured such that, the sidecontacting the wound contact element and the patient's skin may beconsidered adhesive to the skin. Here, adhesive is taken to mean able tostay in place in the absence of negative pressure. Suitable materialsfor the manufacture of a highly conformable transparent wound coveringinclude polyurethanes, polyolefins, polyacrylates, silicone-basedpolymers or composites comprising any combination of these materials.

The wound covering element may be a traditional wound dressing, forexample composed of an absorbent foam, for example, Allevyn (trade mark)made by Smith & Nephew Medical Limited or non-absorbent film such asTegaderm (trade mark) made by 3M Inc.

The wound covering element may be optionally provided with a means ofconnecting the vacuum connection tube with the wound covering elementby, for example, a central or radial aperture or apertures in the woundcovering element. The wound contacting element may be connected, via thewound covering element, to the vacuum connection tube by any means knownto the skilled person including luer fittings such as commerciallyavailable valves and ports, magnetic couplings or adhesive sheet ortape. The connection of the vacuum tube to the wound covering elementmay preferably be achieved via a non-adhesive elastomeric cup positionedat the end of the vacuum connection tube and a pressure-sensitive valvepositioned in the wound covering element itself. The pressure-sensitivevalve may open when a pressure differential of a specified magnitudeexists over its two surfaces such as between 5 and 200 mmHg, forexample. The material comprising the elastomeric cup andpressure-sensitive valve may preferably be a silicone-based material. Itis desirable that the coupling system is suitable for repeated couplingand uncoupling, as convenient for the patient.

Optionally, these coupling elements may be impregnated or coated withantimicrobial materials including, but not restricted to, antibiotics,silver compounds or materials, iodine-based formulations,polyhexamethylene biguanide, triclosan or chlorhexidine. Preferably, theelements may be coated with silver clusters.

The vacuum connection tube may be any of appropriate mechanicalproperties and bore for the transmission of negative pressure from thevacuum source to the wound contacting element. However, dependent uponthe configuration of the wound contacting element, the bore of the tubemay be such that it is incapable of transmitting dry or hydratedcomponents of the wound contacting element. The tubing may be asconformable and light weight as possible and may be coiled or linear.The tubing may be single or multi-lumen, or a combination of lumens, andmay optionally split and or rejoin to form separate tubular elements forthe management of a single wound site or multiple wound sites. Thetubing may be opaque or transparent.

The wound contacting element and wound covering element may beoptionally combined into a single element.

The wound covering element and vacuum connection tube may optionally becombined into a single element.

The wound contacting element, wound covering element and vacuumconnection tube may be optionally combined into a single element.

The vacuum source may be any available and may be optionallymechanically powered by, for example, a compressed spring and comprise asyringe as the vacuum generating means as is known in the prior art; orelectrically powered, for example a vacuum pump. Preferably, the vacuumsource may be capable of generating vacuums in the range of −10 mmHg to−250 mmHg relative to atmospheric pressure. More preferably, the vacuumsource is a vacuum pump capable of generating vacuums in the range of−10 mmHg to −250 mmHg relative to atmospheric pressure. The pump maygenerate a vacuum by any convenient means; diaphragm pumps, peristalticpumps, Venturi-effect pumps and other displacement pumps may be suitablefor this purpose.

The vacuum source may preferably be below 500 g in weight, morepreferably below 100 g in weight and even more preferably below 50 g inweight.

In cases where the vacuum source is electrically powered, the powersource may be mains supplied, a battery supply or a locally generatedsupply such as a clockwork generator, a solar cell, a thermo cell or akinetic autorelay type of power source. Preferably the power source maybe a battery.

In cases where the power source is a battery, the battery may bedisposable or rechargeable. When the battery is rechargeable, rechargingmay be achieved via a charging station for the vacuum housing or for thebattery itself. Battery life may be preferably longer than 12 hours,more preferably longer than 24 hours and even more preferably longerthan 72 hours. The battery may preferably be below 100 g in weight, morepreferably below 50 g in weight.

The power and vacuum sources may be housed separately or together.Preferably they are housed together. When housed, the combined weight ofthe power and vacuum source and housing may be preferably less than 1kg, more preferably less than 500 g, more preferably still less than 200g. The housing may be of any geometry but is preferably adapted so as tobe convenient for the patient and/or carer to handle and carry. It mayalso preferably be of dimensions below 15×15×6 cm.

The apparatus of the first aspect of the present invention may beapplied to wounds for their management. The general principle of theapparatus is the application of the wound contacting element to thewound, covering the wound contacting element with the wound coveringelement and coupling the wound contacting element to a vacuum source viaa vacuum connection tube. As mentioned above, two or more of theseelements may be provided as a single entity. Preferably for largelytwo-dimensional wounds, the wound contacting element and the woundcovering element may be a single entity. This combined entity maycontain the pressure-sensitive valve in its top surface and may beattached to the perimeter of the wound by appropriate means. Attachmentof the dressing to a patient may be achieved by the application ofnegative pressure alone, in the absence of a bonding means or may beachieved via a bonding means. Attachment may preferably be achieved by abonding means, for example a pressure sensitive adhesive applied to theskin contacting surface of the wound covering element. When the bondingmeans is a pressure sensitive adhesive, it may preferably be apoly(acrylate)- or silicone-based formulation.

According to a second aspect of the present invention there is providedapparatus for the application of topical negative pressure therapy to awound site, the apparatus comprising: a wound covering element thatprovides a substantially airtight seal over the wound; a vacuumconnection tube connecting the wound covering element to a vacuumsource; a vacuum source connected to a distal end of the vacuumconnection tube; the wound covering element having valve meansassociated therewith which permits only fluid flow out of a wound cavitydefined by the wound covering element and the wound.

When the wound contacting element and the wound covering element are inplace, the vacuum source is activated and the vacuum connection tube,preferably attached to the vacuum source, may be connected to the woundcovering element via an aperture or valve in the wound covering element.At any point during the application of negative pressure therapy, thecoupling of vacuum connection tube to the wound covering element can bereversibly broken and re-established at the convenience of the patientor carer.

In an embodiment of the apparatus according to either the first orsecond aspects of the present invention the dressing wound coveringelement may comprise a one-way valve as mentioned above, the valveessentially being able to allow fluid in the form of air to be withdrawnfrom the wound cavity defined by the wound covering element via thevacuum connection tube. As mentioned above the vacuum connection tubemay preferably be repeatably connectable and disconnectable to thedressing/wound covering element without damage thereto such that thevacuum source may be removed by the patient and the dressing left inplace but sealed against the ingress of bacteria and potentialinfection, for example, by the presence of the one-way valve in thewound covering element. The one-way valve means may be a simple plasticsmaterial self-sealing valve as are available commercially for manydiverse applications outside of the field of TNP therapy such as thosesold under the trade mark “miniValve” by Mini Valve International, forexample.

Desirably the vacuum connection tube may be attached to thedressing/wound covering element by non-adhesive means so as tofacilitate repeated connection/disconnection thereof without damagingthe wound covering element film material. In this regard the vacuumconnection tube may be connected by “sucker” means at the dressing endof the vacuum connection tube, the sucker means being, for example, inthe form of a cup-shaped, domed or bell-shaped conformable plasticsmaterial member which is in fluid communication with the vacuumconnection tube and which member which may be placed over the valvemeans in the wound covering element and seal with surrounding woundcovering element material, the sucker means being held in place on thedressing/wound covering element by the vacuum generated by the vacuumsource itself. Disconnection of the vacuum connection tube may then beeffected merely by turning off the power source to the vacuum source orby breaking the seal of the sucker by lifting its edge (NB the suckercan easily be removed by this intentional manipulation but cannot easilybe accidentally dislodged by vertical extensive force e.g. pulling onthe vacuum connection tube).

When the wound contacting element becomes saturated with wound exudate,the vacuum connection tube can be disconnected and the wound contactingelement and wound covering element (or a physical combination of thetwo) can be exchanged for a new set.

Wounds suitable for management by the apparatus that is the subject ofthis invention include injuries to soft and hard tissue, including theskin, muscle, cartilage, tendons, bone and internal organs.

In the second aspect of the invention a wound contacting element ofhighly absorbent material may not be present and wound exudate may beaspirated from the wound cavity to a remote waste receptacle by thevacuum source.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be more fully understoodexamples will now be described by way of illustration only withreference to the accompanying drawings, of which:

FIG. 1 shows a schematic cross section of an embodiment of apparatusaccording to the present invention and a wound being treated by TNPtherapy;

FIGS. 2A to 2C which show the arrangement of FIG. 1 simplified and theprogression of saturation of a wound contacting element; and

FIG. 3 illustrates a liquid contact triggered valve.

DETAILED DESCRIPTION OF SOME EXEMPLIFYING EMBODIMENTS

Referring now to the drawings and where the same features are denoted bycommon reference numerals.

FIG. 1 shows a schematic cross section through a wound 10 having anembodiment of apparatus 12 according to the present invention applied toit for the purpose of TNP therapy of the wound. The wound 10 has a woundcontacting element 14 placed in the cavity defined by the wound, thewound contacting element being roughly level with surrounding sound skin16. A wound covering element 18 is applied over the wound to contact andseal with the surrounding sound skin by means of a layer of pressuresensitive adhesive (not shown) coated onto the skin contacting surfaceof the wound covering element material. The wound covering element 18has an aperture 20 in the area above a part of the wound and the woundcontacting element 14. A one-way valve 22 is positioned in the aperture20 to permit fluid in the form of air to be extracted from the woundcavity 24 defined by the wound covering element 18 and the wound 10itself. A vacuum source 26 in the form, in this example, of a batterypowered vacuum pump is connected to the wound cavity by a vacuumconnection tube 28 and a cup-shaped connection member 30 having anaperture 32 therein to accept the end of the vacuum connection tube 28.The cup-shaped member 30 has a flange portion 34 which seats onto theupper surface 36 of the wound covering element 18 material and sealstherewith. The valve 22 has an orifice 40 which is normally closed dueto the resilience of the material from which it is moulded, for example,silicone plastics based material. The orifice 40 is in the form of aslit normally closed by two lips 42, 44 (the shape of the valve orificemay be likened to a flat bladed screwdriver and the valve orifice 40 inFIG. 1 runs normal to the plane of the page). The valve 22 is initiallyheld in and sealed to the wound covering element material 18 by a flangeportion 46 bearing against the underside of the wound covering element18 material and a circular shoulder 48 and recess 50 which is held inthe aperture 20. To evacuate the wound cavity 24, the vacuum pump 26 isstarted and the cup-shaped member 30 placed on the wound coveringelement material above the valve 22 and the reduction in pressure withinthe wound cavity 24 causes the cup-shaped member 30 to be sealedsecurely to the dressing by the excess pressure of the surroundingambient air. As the vacuum in the wound cavity develops, the woundcovering element 18 is pushed down against the wound contacting element14 by ambient air pressure and the wound contacting element 14compressed against the wound surface to apply TNP therapy thereto.

Whilst it is perfectly feasible for the lower surface of the flangeportion 34 of the cup-shaped member 30 to be adhesively coated and to beso retained on the wound covering element material, in this exampleretention of the vacuum connection tube 28 to the wound dressing issolely by ambient air pressure as described above. In case the patientwishes to detach the vacuum pump 26 and leave it behind this may simplybe achieved by turning off the pump 26 and allowing the vacuum todegrade and removing the cup-shaped connection member 30. In this casethe valve 22 is self-sealing prevents access of bacteria and the like tothe wound cavity 24.

The valve 22 exemplified above is a miniValve (trade mark) supplied byMini Valve International. However, this valve is merely exemplary andmany other types of suitable valves are available commercially.

For example International Patent Application No PCT/EP2008/063046discloses a composition which coalesces on hydration and which can beused to provide a valve which closes upon contact with liquid. Theapplication is included fully herein by way of reference but brieflydiscloses a suitable composition that enables a new physicaltransformation. The physical transformation in question involves theconversion of a first stable physical geometry into a second stablephysical geometry upon hydration, wherein hydration enables theself-coalescence (fusion) of spatially separated elements or surfaces ofthe first stable physical geometry.

Each geometry is physically stable. Thus, immersion of the first stablephysical geometry in excess solution results in conversion to the secondstable physical geometry without significant loss of the material massby dissolution. That is, the second stable geometry is insoluble, or hasonly very limited solubility, in the excess solution. The second stablephysical geometry is, at least substantially, self supporting such thatit is able to retain its shape when is excess solution, or when removedtherefrom. In typical preferred forms in the second stable physicalgeometry the material of the invention is a gel or gel like material.

A feature of the composition is the physical homogeneity of the objectin both the first and second physical geometries.

The novel transformation is enabled by construction of the object, atleast in part, from materials that can exist in physically stable formsin the dry state and the hydrated state. Furthermore, the hydrated stateof the material must be sufficiently self-cohesive, even when immersedin excess solvent, to enable fusion to occur. This, we believe, is aproperty unique to a limited range of states of matter, some of which weprepare to exemplify this invention.

In broad terms the composition of matter when formed into an object ofsuitable geometry, can self-coalesce upon hydration in a suitablesolvent.

According to a first aspect of the composition there is provided a highmolecular mass cationic polymer material having a first state whichincludes at least two separate but adjacent surfaces and a second statein which the polymer consists of a homogeneous body, wherein thematerial transitions from the first state to the second state uponhydration.

Thus, on hydration the material expands and the surfaces merge orcoalesce to result in a body of self supporting material, typically agel or gel-like material, which has uniform properties in any dimension.Surfaces and other boundaries within the body of material are absent.Furthermore the body of material is insoluble, or at least of limitedsolubility in the hydrating solvent and is able to retain its physicalgeometry under leading (for example gravity).

The term ‘suitable geometry’ is taken to describe an arrangement whereseparate (for example spatially separate, but not necessarily physicallyseparate) elements or surfaces of the object are sufficiently proximateto enable coalescence upon hydration-induced expansion.

The term ‘suitable solvent’ is taken to describe a fluid (liquid or gas)that can be absorbed be the object, causing expansion and a change inthe physical properties of the object (e.g. surface energy). Thesuitable solvent is typically and preferably an aqueous medium.

The term ‘self-coalesce’ is taken to describe the transformation of twoor more spatially separated physically homogeneous elements into asingle physically homogeneous element or of fusion of previouslyspatially separated surfaces of the same element.

Suitable compositions of matter from which objects can be formed arethose comprised, entirely or in part, of high average molecular weightcationic polymers including zwitterionic (carrying both anionic andcationic charge) polymers with a cationic charge bias. The cationicpolymer may be, or may be a derivative of, a synthetic or a naturallyoccurring polymer. Preferably, the cationic polymer is one carryingamine functionality. More preferably, the cationic polymer is apolysaccharide. More preferably still, the cationic polymer is chitosanor a derivative of chitosan. The chitosan may be derived from anysource, marine or fungal, and is preferably of a weight averagemolecular weight (Mw) exceeding 10 kDa (kilodaltons), more preferablyexceeding 100 kDa and most preferably exceeding 200 kDa.

Where the polymer is a derivative of chitosan, it is preferably acarboxylated derivative. More preferably, it is a carboxyalkylderivative of chitosan. More preferably still, it is a carboxymethylderivative of chitosan. The carboxymethyl derivative of chitosan ispreferably of a weight average molecular weight exceeding 50 kDa, morepreferably exceeding 100 kDa, especially exceeding 500 kDa, moreespecially exceeding 600 kDa and especially 700 kDa or more.

Carboxymethylation is preferably achieved using known reagents: a baseand chloroacetic acid or preferably a neutral salt of chloroacetic acidsuch as sodium chloroacetate. Preferably, the reaction is carried out ina single step: chitosan fibres or (less preferably) particles beingimmersed in a solution of reagents or vice versa. Suitable reactionsolvents include mixtures of an alcohol with water. The alcohol may beany known but is preferably a non-solvent for chitosan andcarboxymethylchitosan, for example isopropanol. The base may be anyknown but is preferably a water-soluble inorganic base such as sodiumhydroxide or potassium hydroxide, preferably sodium hydroxide.

According to a second aspect of the composition there is provided amethod of preparing high molecular mass carboxymethylchitosan comprisingthe steps:

-   -   a. mixing chitosan with a solution of a base and chloroacetic        acid, or a neutral salt thereof, dissolved in a reaction solvent        comprising a mixture of an alcohol and water;    -   b. allowing the reaction to proceed at ambient temperature for        at least 8 hours whilst ensuring adequate exposure of the        chitosan to the reaction solvent;    -   c. when the reaction is complete, washing the reaction product        in excess alcohol-containing solvent;        wherein the volume (in millilitres) of the reaction solvent is        at least 20-times the mass (in grams) of chitosan.

A high molecular mass carboxymethyl chitosan preferably comprises acarboxymethyl chitosan having a mass of at least 500 kDa, moreespecially at least 600 kDa and especially 700 kDa or more.

In one preferred embodiment the volume of reaction solvent (inmillilitres) exceeds the mass of chitosan (in grams) by more than 20 butless than 70-times, more preferably by more than 30-times but less than40-times.

In another preferred embodiment the mass of sodium chloroacetate exceedsthe mass of chitosan by not more than 2-times, more preferably by notmore that 1.2-times.

In a preferred embodiment, the alcohol of the reaction solvent isisopropanol.

In further preferred embodiments the reaction is carried out at ambienttemperature for a period of at least 8 hours, more preferably for atleast 15 hours and even more preferably for at least 18 hours.

In a particularly preferred embodiment, the alcohol of the reactionsolvent is isopropanol, the mass of sodium chloroacetate is not morethan twice (more especially not more than 1.2 times) the mass of thechitosan and the reaction is allowed to proceed for at least 8 hours.

When the chitosan is provided for reaction in powder or fibre form, thismaterial should be adequately exposed to the turbid reaction solventthroughout the duration of the reaction. This process can be facilitatedby any means known to the artisan but can be simply achieved by rollingthe reaction vessel, for example.

When the reaction is complete, reaction by-products detrimental to thestability of the product, such as sodium chloride or sodiumhydroxyacetate, should be removed to the maximum extent feasible. Toachieve this, the reaction product is washed, preferably in one or moresteps, in excess solvent comprised of at least 60 parts alcohol (such asethanol) and 40 parts water (60:40).

More than one washing step is preferred and, when this is the case, thefirst wash step has preferably a higher water content than subsequentsteps, with water content decreasing in every wash step. For example, asuitable two-step wash procedure involves a first wash in excess solventcomprised of at least 60 parts ethanol and 40 parts water (60:40) and asecond wash in excess solvent comprised of at least 90 parts ethanol and10 parts water (90:10).

Thus in a preferred embodiment the reaction product is washed in aplurality of washing stages, each employing an excess of a solventcomprising alcohol and water, wherein in each succeeding stage thesolvent consists of a higher proportion of alcohol. Preferably thealcohol is ethanol

It is essential that wash solvents always includes some water to avoidexcessive dehydration of the product, which can result in brittleness.

The composition of the wash solvent may include any suitable alcoholssuch as ethanol, isopropanol or methanol. Ethanol is preferred.

The product resulting from washing and solvent removal can be sterilisedby methods typical for the sterilisation of medical devices, for examplegamma-irradiation, electron-beam irradiation or ethylene oxidetreatment.

Prior to radiation-based sterilisation, the washed reaction productshould be adequately solvent-free. This can be achieved by any dryingprocess known to the skilled artisan. A preferred drying process isconducted at temperatures not exceeding 40° C., more preferably notexceeding 30° C. Preferably, solvent removal is achieved by placing thematerial under a sub-atmospheric pressure. The pressure is preferablyless than 500 mbar, more preferably less than 1000 mbar. The duration ofthe drying process, when achieved by vacuum drying, preferably exceeds 8hours, more preferably exceeding 12 hours.

The weight average molecular weight of the material following washingand radiation sterilisation is preferably greater than 120 kDa, morepreferably greater than 130 kDa and after washing and ethylene oxidesterilisation is preferably greater than 400 kDa, more preferablygreater than 500 kDa. It is important that these molecular weights areobtained to avoid mechanical integrity problems in the final product anddissolution problems when exposed to fluid.

Additives and co-components can be added at any stage of the aboveprocess, prior to terminal sterilisation. These agents may be anysuitable for a topical or internal medical application, such asanalgesics, anaesthetics, antimicrobial agents, anti-cancer agents,nicotine or nicotine substitutes or other synthetic or naturally-derivedpharmaceuticals including peptides, proteins such as growth factors orretardants, enzymes (e.g. those facilitating tissue debridement), DNA orRNA fragments.

When the additive is an antimicrobial agent, it may be for example:silver or silver compounds, iodine or iodine compounds, quaternaryamine-based antimicrobials such as polyhexamethylenebiguanide orchlorhexidene, antibiotics such as gentamycin, vancomycin or apeptide-based agent.

When silver is introduced into the formulation, and the formulation iscarboxymethylchitosan-based, addition is preferably achieved byimmersion in a solvent mixture of a similar composition as that appliedduring the carboxymethylation process.

In a third aspect, the composition can be used to provide a method offusing two or more solid surfaces, wherein the surfaces are initiallyseparate (in particular, spatially separated) but adjacent surfaces ofone or more object(s) comprising a self-coalescing material as hereindescribed, notably the high molecular mass polymer material of the firstaspect of the invention. The method comprises the step of immersing saidsurfaces in an aqueous medium and thus hydrating and expanding theself-coalescing material. In one embodiment, the surfaces are initiallyspatially separated surfaces of the same object. Alternatively, thesurfaces are initially spatially separated surfaces of differentobjects. These alternatives are not mutually exclusive. The surfaces maybe the surfaces of fibres, for example in a woven or, more especially, anon-woven fibrous material. In such materials, the surfaces may haveportions which are spaced apart and portions which, while beingseparate, are in contact.

Objects fabricated from the compositions defined above, and suitable forthe method, need to be suitably designed to enable coalescence uponhydration. For example, an isolated linear object would not have theopportunity to self-coalesce upon hydration. In contrast, a pair ofisolated but adjacent linear objects would have the opportunity to swelland coalesce upon hydration. In this context, ‘adjacent’ means locatedwithin about 10 mm of one another. Thus, suitable objects can be definedas containing, at least in part, spatially separated elements orsurfaces located within about 10 mm of one another. Preferably, thespatially separated elements or surfaces are located within 5 mm of oneanother. More preferably, the spatially separated elements or surfacesare located within 1 mm of one another. In some cases, for example fibrebased materials, at least parts of adjacent surfaces may be in contact.

Preferred physical formats that meet the above description arefibre-based materials such as woven and non-woven materials. Othersuitable formats include knits, open-celled foams and laminatesincluding corrugated materials. More complex arrangements can befabricated by methods known to one skilled in the art, such aslithography, micromachining and electrospinning. The composition and itsuses is not restricted to formats of high open area but includes solidmonoliths. Fibre based materials are preferred and fibre-based non-wovenmaterials are particularly preferred.

The composition in use is not restricted to objects consistingexclusively of self-coalescent material, but includes composites, forexample composites of common medical device formats and self-coalescentmaterial and surface-coatings, for example implantable metal- orbiomaterial based devices including soft-tissue substitutes and jointimplants. Composites suitable for topical and internal wound managementinclude those combining polyurethane based materials, such as foams,slabs and films with self-coalescent materials, for example in powderedor, more especially, fibrous form.

When devices comprised, at least in part, of the compositions areimmersed in a fluid, they absorb fluid, become swollen and self-coalesceacross contact points. Use is not restricted to specific compositions orspecific fluids, but in preferred forms and for preferred end-sues, thefluid is most preferably water based. For example, in the case ofcarboxymethylchitosan-based materials, the fluid is preferably waterbased. Examples of water based fluids include water or a solution ofwater, such as saline or a biologically-derived fluid such as wholeblood, blood plasma, serum, saliva, wound exudate or bone marrowaspirate.

The novel material properties of the described self-coalescing materialscan be exploited in a range of applications, for example in irreversiblefluid valving systems and moulding materials.

EXAMPLES Example 1 Generation of Self-Coalescing CarboxymethylchitosanFibres A) Synthesis

Immediately prior to reaction, sodium chloroacetate (1.75 g) wasdissolved in 4% aqueous sodium hydroxide solution (7 ml). This solutionwas added to isopropanol (45 ml) and shaken vigorously, resulting in aturbid suspension. This mixture was added to a vessel containingchitosan fibres (1.50 g), the container sealed and rolled atapproximately 60 rpm for 18 hours.

B) Washing Steps

B1) After step A, the fibres were removed from the now clear reactionsolvent and transferred to a vessel containing 99:1 ethanol:water (200ml). The material was disturbed every 15 minutes for 1 hour, after whichtime the material was removed and physically dried by the application ofhand pressure between several layers of absorbent material. Followinggross drying, the material was vacuum dried at ambient temperatureovernight.

B2) After step A, the fibres were removed from the now clear reactionsolvent and transferred to a vessel containing 60:40 ethanol:water (200ml). The material was disturbed every 15 minutes for 1 hour, after whichtime the material was removed and transferred to a second vesselcontaining 90:10 ethanol:water (200 ml). The material was disturbedevery 15 minutes for 1 hour, after which time the material was removedand physically dried by the application of hand pressure between severallayers of absorbent material. Following gross drying, the material wasvacuum dried at ambient temperature overnight.

Example 2 Generation of Self-Coalescing Carboxymethylchitosan Fibres(Scale-Up)

Immediately prior to reaction, sodium chloroacetate (96.8 g) wasdissolved in 4% aqueous sodium hydroxide solution (387 ml). Thissolution was added to isopropanol (2490 ml) and shaken vigorously,resulting in a turbid suspension. This mixture was added to a vesselcontaining chitosan fibres (83.0 g), the container sealed and rolled atapproximately 60 rpm for 18 hours. After this time, the fibres wereremoved from the now clear reaction solvent and transferred to a vesselcontaining 99:1 ethanol:water (2000 ml). The material was disturbedevery 15 minutes for 1 hour, after which time the material was removedand physically dried by the application of hand pressure between severallayers of absorbent material. Following gross drying, the material wasvacuum dried at ambient temperature overnight.

Example 3 Radiation Sterilisation of Self-CoalescingCarboxymethylchitosan Fibres

The material resulting from Example 1, step B2 was packaged ingas-permeable sterilisation pouches and sterilised by gamma irradiationat 30-40 kGy. The molecular weight of the material pre- andpost-sterilisation was determined by gel permeation chromatography. Themolecular weight prior to sterilisation was approximately Mw 700 kDa;the molecular weight post-sterilisation was approximately Mw 140 kDa.The molecular weight change in the material, although substantial, wassuch that the physical properties of the material were not significantlyaltered by sterilisation.

Example 4 Ethylene Oxide Sterilisation of Self-CoalescingCarboxymethylchitosan Fibres

The material resulting from Example 1, step B2 was packaged ingas-permeable sterilisation pouches and sterilised by ethylene oxidetreatment. The molecular weight of the material pre- andpost-sterilisation was determined by gel permeation chromatography. Themolecular weight prior to sterilisation was approximately Mw 700 kDa;the molecular weight post-sterilisation was approximately Mw 575 kDa.The molecular weight change in the material was such that the physicalproperties of the material were not significantly altered bysterilisation.

Example 5 Water Absorbency of Self-Coalescing CarboxymethylchitosanFibres

The material resulting from Example 3 (100 mg) was immersed in water (4ml) for 1 minute and withdrawn. Excess liquid was allowed to drain andthen the hydrated transparent mass was weighed. The material wascalculated to absorb approximately 25-times its own mass in waterwithout significant dissolution.

Example 6 Serum Absorbency of Self-Coalescing CarboxymethylchitosanFibres

The material resulting from Example 3 (100 mg) was immersed in serum (4ml) for 1 minute and withdrawn. Excess liquid was allowed to drain andthen the hydrated transparent mass was weighed. The material wascalculated to absorb approximately 13-times its own mass in serumwithout significant dissolution.

Example 7 Self-Coalescence of Carboxymethylchitosan Fibres in Water

The material resulting from Example 3 (100 mg) was immersed in water (4ml) for 1 minute and withdrawn. Excess liquid was allowed to drain andthen the hydrated transparent mass was allowed to stand for 4 hours.After this time, the individual fibres of the material hadself-coalesced and the material was then effectively a homogeneous,elastic hydrogel, able to stably retain its physical geometry underloading (FIG. 2).

Example 8 Self-Coalescence of Carboxymethylchitosan Fibres in Serum

The material resulting from Example 3 (100 mg) was immersed in serum (4ml) for 1 minute and withdrawn. Excess liquid was allowed to drain andthen the hydrated transparent mass was allowed to stand for 4 hours.After this time, the individual fibres of the material haveself-coalesced and the material as effectively a homogeneous, elastichydrogel, able to stably retain its physical geometry under loading.

FIGS. 2A to 2C show stages in the absorption of wound exudate into thewound contacting element 14. The basic arrangement may be the same asthat of FIG. 1, however, the drawings of FIG. 2 have been simplified.FIG. 2A shows a clean dressing newly installed in the wound 10 andwithout any exudate being taken up; FIG. 2B shows the wound contactingelement 14 partially full of exudate; and, FIG. 2C shows the woundcontacting element 14 saturated with wound exudate. However, the natureof the wound contacting element material 14 is such that no liquid isaspirated by the vacuum connection tube 28 out of the wound cavity 24 orindeed out of the wound contacting element 14 itself.

FIG. 3 shows paradigms of the conversion of a first stable physicalgeometry into a second stable physical geometry upon hydration, whereinhydration enables the self-coalescence (fusion) of spatially separatedelements of the first stable physical geometry. In case A, theself-coalescence comprises the fusion of spatially separated surfaces ofa single element; in case B the surfaces are adjacent surfaces of twoseparate elements.

The Examples described below are based on the use of the apparatusarrangement shown in FIG. 1 and/or FIG. 2 and/or FIG. 3.

Example 1

Application of apparatus of FIG. 1 to ex vivo cavity wound. A 5 cmdiameter, 5 cm depth cavity wound was cut by scalpel into a porcine legjoint. The musculature in the area of the wound cavity was injected withsaline to ensure that the tissue was adequately hydrated for theduration of the experiment. The wound cavity was packed with two woundcontacting elements of non-woven balls of carboxymethylchitosan fibreand the wound cavity and filling was covered over with an adhesivesilicone gel sheet CicaCare (trade mark) made by Smith & Nephew MedicalLtd with a central 5 mm diameter aperture 20. A luer loc fittingattached to a coiled vacuum hose was inserted through the aperture andconnected to a battery-powered vacuum source Pac-Vac (trade mark) madeby Virtual Industries Inc. Immediate contraction of the wound margin wasobserved and the non-woven balls were compressed down to be flush withthe surface of the skin. The system was left in place for 8 hoursundisturbed, after which time no fluid had exited the cavity packing buthad collected within it.

Example 2

Assembly of apparatus based on FIG. 1 having a wound covering element 14of Allevyn Adhesive (trade mark) made by Smith & Nephew Medical Ltd.

A 3 mm diameter aperture in the top film of Allevyn Adhesive was createdusing a biopsy punch. Over this aperture was bonded a silicone elastomerdome miniValve (trade mark) made by Mini Valve International B. V., partnumber DO 072.004. A miniature vacuum source made by Virtual IndustriesInc., PAC-VAC V3200 (trade mark) was coupled by vacuum tubing 28 to ahand-made silicone rubber cup 30.

Example 3

Usage of apparatus based on Allevyn Adhesive as in Example 2. A 5 cmdiameter, 5 cm depth cavity wound was cut by scalpel into a porcine legjoint. The musculature in the area of the wound cavity was injected withsaline to ensure that the tissue was adequately hydrated for theduration of the experiment. The wound cavity was packed with woundcontacting element 14 comprising two non-woven balls ofcarboxymethylchitosan fibre and the wound cavity 24 and filling wascovered over with the modified Allevyn Adhesive dressing described inExample 2. The vacuum source, as described in Example 2, was turned onand the vacuum tubing cup placed over the dome valve positionedcentrally on the Allevyn Adhesive. Immediate contraction of the AllevynAdhesive dressing and the wound margin was observed and the non-wovenballs were compressed down to be flush with the surface of the skin. Thesystem was left in place for 8 hours undisturbed, after which time nofluid had exited the cavity packing but had collected within it.

Example 4

Usage of apparatus based on Allevyn Adhesive as described in Example 2.A 5 cm diameter, 5 mm depth shallow wound was cut by scalpel into aporcine leg joint. The musculature in the area of the wound was injectedwith saline to ensure that the tissue was adequately hydrated for theduration of the experiment. The wound was covered with a non-woven sheetof carboxymethylchitosan as the wound contacting element 14 and thewound and non-woven sheet was covered over with the modified AllevynAdhesive dressing described in Example 2 as the wound covering element18. The vacuum source, as described in Example 2, was turned on and thevacuum tubing cup placed over the dome valve positioned centrally on theAllevyn Adhesive. Immediate contraction of the Allevyn Adhesive dressingwas observed. The system was left in place for 8 hours undisturbed,after which time no fluid had exited the wound dressing but hadcollected within it.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of the words, for example“comprising” and “comprises”, means “including but not limited to”, andis not intended to (and does not) exclude other moieties, additives,components, integers or steps.

Throughout the description and claims of this specification, thesingular encompasses the plural unless the context otherwise requires.In particular, where the indefinite article is used, the specificationis to be understood as contemplating plurality as well as singularity,unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties orgroups described in conjunction with a particular aspect, embodiment orexample of the invention are to be understood to be applicable to anyother aspect, embodiment or example described herein unless incompatibletherewith.

1-25. (canceled)
 26. A method for the application of topical negativepressure therapy to a wound site, the method comprising: positioning anabsorbent material and a flexible wound covering over the wound site,the flexible wound covering being positioned over the absorbent materialand the flexible wound covering being roughly level with surroundingskin; applying negative pressure to the wound site through a conduitconnected to the wound covering and through an aperture in the woundcovering, wherein a liquid blocking element is positioned in theaperture of the wound covering; retaining fluid from the wound site inthe absorbent material; and preventing substantially all liquid from thewound site from entering the conduit with the liquid blocking element,wherein the liquid blocking element blocks substantially all liquidtransport beyond itself away from the wound site.
 27. The method ofclaim 26, wherein preventing substantially all liquid from the woundsite from entering the conduit comprises operating a valve positioned atthe aperture of the wound covering.
 28. The method of claim 26, whereinthe absorbent material and the wound covering are positioned over thewound site as a single element.
 29. The method of claim 26, wherein theabsorbent material is configured to transform into a gel upon contactwith wound exudate.
 30. A method for the application of topical negativepressure therapy to a wound site, the method comprising: positioning awound therapy apparatus over a wound site, the wound therapy apparatuscomprising: a wound covering element configured to be positioned overthe wound site, wherein the wound covering element comprises anaperture, and an absorbent material positioned in or over the aperture,the absorbent material configured to transform into a gel upon contactwith wound exudate; and applying negative pressure to the wound sitethrough a conduit connected to the aperture in the wound covering. 31.The method of claim 29, wherein the absorbent material comprises aself-coalescing material.
 32. The method of claim 29, wherein the gel isconfigured to absorb wound exudate.
 33. The method of claim 29, whereinthe gel comprises uniform properties in each dimension.
 34. The methodof claim 29, wherein the gel is a cellulosic gel.
 35. The method ofclaim 29, wherein the absorbent material comprises a polyanionicpolymer.